In vitro studies have shown that non-dividing blood lymphocytes from patients with chronic lymphocytic leukemia are more sensitive than normal lymphocytes to the cytocidal action of many reagents including prednisolone, L-asparaginase, cytarabine, vincristine, and colchicine. As the maximum difference was obtained with colchicine, a pilot study was undertaken to consider the possibility of using this reagent as a special slide chamber and the counting of viable lymphocytes by phase contrast microscopy. The sensitivity was expressed as the average cytocidal effect produced by 1.0 and 0.1 ug/ml of colchicine incubated with the cells at 37 degrees C for 20 hours. A high degree of sensitivity of blood lymphocytes was observed for patients with active chronic lymphocytic leukemia; low sensitivity was obtained for normal persons. The cytocidal effect of colchicine on leukemic lymphocytes suggests hypothesis that the reagent acts on microtubules of the non-dividing cells. This hypothesis is being tested by measuring the effect of D2O on the cytotoxicity of colchicine.